Calutron shielding



Nov. 29, 1955 J. R. RICHARDSON CALUTRON SHIELDING 4 Sheets-Sheet l Filed June 14, 1946 Av1/avro@ dof/N /P. /Q/CHARoso/v A rToR/vfx NOV. 29, 1955 J. R. RICHARDSON CALUTRON SHIELDING 4 Sheets-Sheet 2 Filed June 14, 1946 /m/f/vro@ JOHN A. /P/c//A/QOJON @y NOV- 29, 1955 J. R. RICHARDSON 2,725,480

CALUTRON SHIELDING Filed June 14, 1946 4 Sheets-Sheet 5 Vacuum Em/e/ope? [.37 /5 JO/fl( Power Supp/y ATTOR/VEK Nov. 29, 1955 J. R. RICHARDSON CALUTRON SHIELDING 4 Sheets-Sheet 4 Filed June 14, 1946 ArroR/VEK ttes Patet 2,725,480 Patented Nov. 29,v 1955 hcc CALUTRON SHIELDIN G John R. Richardson, Berkeley, Calif., assignor to .the United States of America as represented by the United States Atomic Energy Commission Application June 14, 1946, Serial No. 676,615

2 Claims. (Cl. Z50-41.9)

This case relates to calutrons and more particularly to a device for the removal of oscillating electrons customarily found in the region of parts of a calutron maintained at a positive potential.

Calutro-ns are described generally in Atomic Energy for Military Purposes by H. D". Smyth, and in greater detail in the copending application of Ernest O. Lawrence, Serial No. 557,784, filed October 9, 1944, now Pat. No. 2,709,222. ln general, a polyisotopic beam of ions is projected into a magnetic iield wherein the ions travel in curved paths, the paths of the lighter ions being of smaller radius than the heavier ones, enabling collectors to be placed across the different paths at a convenient point, such as the 180 of travel along their arcuate paths. Calutrons have been principally employed., and very successfully employed, in the separation of theisotopes of the element uranium, the isotope of mass 235 being separated from the isotope of mass 238. Calutrons, however, can be employed for separating commercial quantities of isotopes of practically all of the elements.

In the operation of the calutrons, electric tields are employed to cause the charge particles, the ions, to be proiected into a magnetic ield. Accordingly, many parts of the calutrons are maintained at positive potentialsfwith respect to the vacuum tank in which they are placed, the vacuum tank being operated at ground potential for safety reasons. Electrons are liberated incident to the operation of the calutron and tend to move toward these positive members. Due to the fact, however, that a strong magnetic Iield passes through all of the parts of a calutron, the movement of the electrons toward the positive bodies is retarded, resulting in violent oscillations of the electrons along the magnetic iield lines. These violent oscillating electrons possess considerable energy'and in their migration about the positive bodies strike structural members such as insulators heating them locally and causing them to crack and fail- Further, they strike many metal parts of the calutron causing them to be melted. and otherwise damaged.

Means have been discovered, however, to limit the area in which these electrons may oscillate, employing the use of confining electrical shields about the members maintained at a positive potential. Further, the shields are curved transversely to the magnetic eld and tins project therein to further reduce the volume within which the electrons oscillate. These structures are -described in the copending application of Ernest O. Lawrence, Serial No. 552,544, tiled September 2, 1944, but they do not operate to remove these oscillating electrons, but rather only confine the space in which they may oscillate. Several schemes have been devised for removing these electrons completely, that is, discharging them so that they are no longer discrete particles, and the present invention relates to such a device. In general, the present invention is a pair of iin and blister arrangements, aspreviously described, but so positioned with respect to each other and with respect to the magnetic field that theyv completely remove the oscillating electrons, so that they cannot even within a limited area cause damage.

It is therefore an object of the present invention to provide means for removing oscillating electrons that are customarily found in calutrons.

Another object of the invention is to provide a pair of iin and blister arrangements that act to discharge or remove oscillating electrons.

Still a further object of the invention is to provide electron removal devices for a calutron ion generator that is maintained at a positive potential with respect to the vacuum tank in which it is positione Other objects and advantages of the invention will be apparent from the following description and claims t0- gether with the drawings forming a part of the specification, in which:

Figure l is a schematic plan view, partly in section, of a calutron wherein the magnetic field is vertical;

Fig. 2 is a view along the line 2 2 of Fig. 1, showing details of the ion generator;

Fig. 3 is an isometric view of a complete calutron source unit to which the invention has been applied, and including the removable face plate of the vacuum envelope;

Fig. 4 is a schematic view of the pri-ncipal electrical components of the source of Fig. 3 wherein the applied voltages are indicated by different types of linesl outlining the components;

Fig. 5 is,v a view of the forward end of the source of Fig. 3 taken along a horizontal transverse plane; and

Fig. 6 is a View of the fin and blister arrangements incorporating the invention and taken along the line 6-6 of Fig. 3.

Referring to the schematic showing of Figs. 1 and 2, there is shown a lower magnet pole face 200 and an upper magnet pole face 201 between which a magnetic field is generated by any means, such as electromagnetic means, and a vacuum tank 202 is disposed therein, suitable pumping means being connected to an outlet 203 to maintain the pressure therein at a desirable level,` such as 10-4 to 10-5 mm. Hg. Supported on a removable face plate 204 of the vacuumv tank is an insulator 205- which supports on its inner end an ion generator 206. The ion generator is maintained at a high positive potential by suitable means and generates positive ions of the desired element, such as uranium, and these are projected through the ion exit or are slit opening 210 into a magnetick field by a high negative potential maintained on theV accelerating electrodes 207. inasmuch as the intense electric field set up between the ion generator 206 and the electrodes 207 gives the positive ions too much energy for a. desired radius of curvature, decelerating electrodes 208 are provided that are maintained at ground potential, thus reducing the energy of' the ions. The electric fields are designated generally by the equipotential lines 221. These ions are projected into the magnetic field permeating the vacuum tank 202 and separate into two paths 212 and 213, heavier ions U238 taking the path 212 of larger radius and the lighter ions U235 taking the path 213 of smaller radius. Supported on a removable face plate 214 is a collector 216 having two separate apertures 217 and 21S for the collection of the respective isotopes.

Referring again to the ion generator, it will be noted that a sheet of metal 209 is placed on one side of the ion generator 206 and is supported by the vacuum tank 202 by a bracket 212i. This sheet of metal 209 serves to confine on one side the electric eld surrounding the positive ion generator, since otherwise the electric field would extend throughout the vacuum tank to the walls of the vacuum tank. When theA positive ions are projected into the magnetic eld, they collide with molecules or" the ambient atmosphere therein, liberating large quantities of electrons in the space between the ion generator 206 and the accelerating electrodes 207. These liberated electrons try to reach the positive ion generator 206, but because of the strong magnetic iield that permeates the entire vacuum tank, the paths of travel ion beams are curved, but in an oppositel direction and with an infinitely smaller radius because of their great diierence in weight compared to the uranium ions. Thus, the electrons travel for a while toward the ion generator and then curve away from it until they reach an equipotential line that reflects them again back toward the ion generator. Thus, in a magnetically transverse plane, the electrons deline a scalloped path 219, the arrows of which indicate the direction of electron travel. In the plane along the magnetic eld, however, these electrons are in violent oscillation. This is indicated more clearly in Fig. 2 wherein arrows along one of the equipotential lines 221 show the repelling influence of the marginal equipotential lines. Thus, electrons in the space between the ion generator 206 and the grounded shield 209 are attracted toward the ion generator 206, but because of the confining influence of the magnetic field are projected along the magnetic iield until they reach the upper or lower end of the ion generator 206 where the equipotential lines are curved.

In this region of curved lines, they will progress along the magnetic iield until they encounter a line suiciently negative to repel them back in the direction from whence they came. This action occurs at the top and bottom of the ion generator 206 resulting in the attraction and then repulsion, then attraction and then repulsion, until an extremely high frequency of oscillation is attained. It is the purpose of the present invention to discharge these oscillating electrons and thus remove them entirely so that they cannot do structural damage.

The present invention will be described with reference to a calutron ion source on which it was first applied. This calutron source is shown in Fig. 3 in its completely assembled condition. There is illustrated a removable face plate for the vacuum envelope, to which is secured a supporting bracket 16. The bracket 16 is preferably cast and provides a supporting platform for the ion generator, and the accelerating and the decelerating electrodes, and is generally transverse to the magnetic ield, which is vertical with respect to Fig. 3. As a general measure of the size of this particular embodiment of the invention, it is noted that in one operative embodiment the bracket 16 is roughly about three feet in length and a foot and a half wide. Supported by bracket 16 is an ion generator unit 17 that is held at a high positive potential with respect to the face plate 15 and the supporting bracket 16, which are at ground potential. The ion generator 17 is surrounded about its sides by spaced shielding 1S, held at ground potential and therefore referred to as grounded or cold shielding. Formed in the grounded shielding 18 is a iin projects, as will be described later. Cooperating with blister 144 and its associated iin in accordance with the invention is a lower and more rearwardly disposed blister 146 also having an associated iin. Potential is supplied to the ion generator 17 by hot leads 24 which pass through an aperture 23 in the face plate 15 and through an aperture in the grounded shielding 1S. A tubular portion 25 of the grounded shielding 18 projects into the aperture 23 and surrounds the hot leads 24. The accelerating electrode is not visible in Fig. 3; however, a decelerating electrode 19 does appear in this view. This decelerating electrode 19 is heated electrically, one terminal for the heater supply being a strip 22 secured to the forward end of bracket 16, the other end being grounded to the decelerating electrode 19, porting bracket 16 is cooled by a water tube 21 of copper or other suitable material, laid about its inside edges. All parts of the ion source are preferably of a nonmagnetic material to prevent local disturbances of the magnetic field passing therethrough.

are curved, just as the blister 144 into which a Y which is at ground potential. The supi The electrical construction and operation of the entire source unit is shown in Fig. 4, wherein are shown bottom and top walls 26 and 27 of a vacuum envelope, as well as the face plate 15 that completes the vacuum enclosure. The various parts of the ion source unit which are at ground potential are indicated by solid lines, the various insulators are indicated by stippled surfaces, the parts of the insulators at a high positive potential are indicated by dash-dot lines, and the parts of the ion source at negative potential are indicated by broken lines. A schematic power supply 43 is shown which may be of any suitable capacity, such as 50 kv. A resistor 44 connects the output terminals and at any desired point this resistor may be connected to ground, as by a lead 48, so that one end of the power supply 43 is negative with respect to ground and the other end positive.

The ion generator 17 is supported on an insulator 23 of the pedestal type by means of a T -shaped bracket 29. There is thus provided a physical clearance between the ion generator 17 and all other parts of the ion source. The ion generator 17 is held at a high positive potential, as indicated by the dash-dot lines, by a lead 34 connected thereto and passing through the face plate 15 into a tubular elbow 36 which has a bushing-type insulator 37 secured to its lower end. The lower end of the bushing insulator 37 is closed by a plate 38, to which a positive lead 47 from the power supply 43 is connected. A cathode assembly 33 is also shown in dot-dash lines, inasmuch as this structure varies from the potential of the ion generator 17 by only a few hundred volts, and the leads are similarly brought through the elbow tube 36. Also shown in Fig. 4 is an accelerating electrode 31 which is suitably apertured as indicated in Fig. 5, so that the ions from the generator 17 may pass therethrough. This accelerating electrode is supported on the two post insulators 32 by an adjustable mechanism. A lead 39 for the accelerating electrode passes through the face plate 15 by means of a transformentype insulator 41, supported on a tubular collar 42 secured to the face plate 15. A lead 46 connects the outer end of insulator 41 with the negative terminal of the power supply 43. The decelerating electrode 19 is at ground potential, as indicated by the solid lines, and needs no electrical lead because it is mechanically fastened to the grounded bracket 16, which in turn is secured to the grounded face plate 15.

The general description of the ion source is continued with reference to Fig. 5. There it will be noted that the ion generator 17 includes a reservoir 71 which is suitably heated and into which a charge container 72 may be inserted. The application of heat causes the solid charge, such as uranium tetrachloride, to vaporize, and the vapor ows through a nipple 74 into a mixing chamber 76 where it is distributed and somewhat confined by a double-knifed edge baiiie plate 77. Thereafter, it passes into an arc chamber 78 where it is bombarded by an electron stream from an electron ernissive filament (not shown). The positive ions and the un-ionized vapor thereafter issue through the arc slit opening 81, the positive ions being accelerated by the high negative potential upon the accelerating electrodes 31. After passing through the apertured electrode 31, the ions are subjected to a decelerating electric ield deiined by an apertured plate 82 having rounded aperture edges and mounted in the decelerating electrode structure 19. A decelerating electric iield is present due to the fact that the decelerat ing electrode 19 is held at ground potential, which is positive with respect to the accelerating electrode 31. Typical voltages that may be applied to the electrode structure are a positive kv. on the ion generator 17 with respect to ground, a negative l5 kv. on the accelerating electrode 31 with respect to ground resulting in a total accelerating eld of kv., and a decelerating po tential at ground resulting in a decelerating iield of l5 kv., giving the beam a net energy as though accelerated by a single electrode of 35 kv. The accelerating field of 50 kv., however, produces an ion beam of much greater intensity than a simple single electrode at 35 kv. and this is the principal reason for having the two sets of electrodes.

In operation, the vapor emanating from the heated reservoir 72 is bombarded in the arc chamber 78 by an electron stream forming an arc discharge, the plasma of this arc discharge is a copious supply for positive ions, and these are accelerated through the electrode 31 by the electric field maintained between the ion generator 17 and the electrode 31. Thereafter, they are decelerated in passing through the decelerating electrode structure 19. These ions strike gas particles along the route liberating electrons, and in the region between the ion generator and the accelerating electrode 31, these electrons are attracted by the positive potential maintained on the ion generator 17. In seeking to reach the ion generator, they describe the scalloped paths as detailed with reference to Fig. l and at the same time oscillate violently along the magnetic field. In their migration along their scalloped paths, they enter the region between the grounded shielding 18 and the ion generator 17. A tin 141 projects into the blister 144 mentioned first with reference to Fig. 3. These projecting fins break up the electric field that otherwise would permit oscillation of the electrons along the entire side of the ion generator 17. The oscillation therefore is confined to a very small space between the end of the fin and the inside of the blister, as best shown in Fig. 6, the fin being positive and the portions of the blister above and below being negative. This oscillation volume 145 is shown in Fig. 6 by hatch lines. This confinement of the oscillations to such a small volume is in accordance with the second Lawrence application mentioned previously. However, it will be appreciated that a single fin cooperating with its respective blister cannot remove the electrons because a space is to be kept entirely around the fin to prevent electrical breakdown between the high positive ion generator and the grounded shielding. Therefore, the electrons merely migrate around the fin 141 while maintaining their ver",l

limited oscillatory motion. This is indicated by the scalloped line 150 shown in Fig. 5.

In accordance with the present invention, however, a lower fin 142 is provided that is placed underneath and overlapping the end of fin 141. Thus, as the electrons migrate around the end of the upper fin 141, they come into the space between the grounded shield 18 and the ion generator 17 in which there isno longer an electric field that will permit them to oscillate. The lower fin offers them only a positive potential, directly along the lines of magnetic ux and they take the path 155 (Fig. 6)

positive calutron element and 6 I and discharge directly to this fin 142. Accordingly they are discharged and completely removed from the region between the shield and the ion generator. Thus, the present invention provides means to completely remove these electrons, thereby protecting all structural parts that are magnetically transverse and lead to the vacuum wall.

While this invention has been described with reference to the colutron ion generator, it is not limited thereto and obviously could be applied to any body in a calutron that is subject to electron oscillation, for example, a receiver might for various constructions be maintained at a positive potential. Nor is the invention limited to calutrons or otherwise except by the terms of the following claims.

What is claimed is: l

l. A calutron comprising means for establishing a magnetic field, a vacuum tank disposed therein, an ion generator disposed within said tank and maintained at a positive potential with respect to the vacuum tank, a metal shield disposed along the lines of the magnetic field and closely adjacent to the ion generator maintained at the potential of the vacuum tank, a plurality of hollow protuberances on the grounded shield projecting away from the ion generator and overlapping each other with respect to a direction along the magnetic field, and a corresponding plurality of magnetically transverse fins secured to and maintained at the potential of the ion generator and projecting into their respective protuberances and overlapping each other with respect to a direction along the magnetic field.

2. in a calutron, means for establishing a magnetic iield, a vacuum tank disposed therein, a calutron element disposed within the tank and maintained at a positive potential with respect to the tank, a shield placed adjacent to the positive calutron element and maintained at the potential of the vacuum tank, a plurality of hollow protuberances on the shield projecting away from the positive calutron element and overlapping each other both with respect to a direction along the magnetic field, and with respect to a direction extending from front to rear of said calutron element, and a corresponding plurality of magnetically transverse fins that overlap each other with respect to a direction along the magnetic iield, said fins being secured to and maintained at the potential of the projecting into their respective protuberances.

References Cited inthe le of this patent UNITED STATES PATENTS 2,486,452 Washburn et al. Nov. 1, 1949 

1. A CALUTRON COMPRISING MEANS FOR ESTABLISHING A MAGNETIC FIELD, A VACCUUM TANK DISPOSED THEREIN, AN ION GENERATOR DISPOSED WITHIN SAID TANK AND MAINTAINED AT A POSITIVE POTENTIAL WITH RESPECT TO THE VACUUM TANK, A METAL SHIELD DISPOSED ALONG THE LINES OF THE MAGNETIC FIELD AND CLOSELY ADJACENT TO THE ION GENERATOR MAINTAINED AT THE POTENTIAL OF THE VACUUM TANK, A PLURALITY OF HOLLOW PROTUBERANCES ON THE GROUNDED SHIELD PROJECTING AWAY FROM THE ION GENERATOR AND OVERLAPPING EACH OTHER WITH RESPECT TO A DIRECTION ALONG THE MAGNETIC FIELD, AND A CORRESPONDING PLURALITY OF MAGNETICALLY TRANSVERSE FINS SECURED TO AND MAINTAINED AT THE POTENTIAL OF THE ION GENERATOR OVERLAPPING EACH OTHER WITH RESPECT TO A DIRECTION ALONG THE MAGNETIC FIELD. 